Different from customary analog contents, digital contents of music and so forth can be copied by a plural number of times without suffering from any deterioration of the quality. Therefore, in recent years, illegal distribution and exchange of contents which are not permitted by the owner of the copyright have been and are increasing together with the popularization of the Internet and increase of the speed and the capacity of PCs (Personal Computers) and so forth.
In order to prevent such illegal acts, a copyright management system which makes use of a DRM (Digital Rights Management) technique of restricting the distribution and utilization of contents is being popularized. In the copyright management system mentioned, it is a popular method that the number of copies of an original content is managed strictly to restrict illegal copying as proposed by the SDMI (Secure Digital Music Initiative) and so forth (refer to, for example, Patent Document 1 (Japanese Patent Laid-Open No. 2003-296486)).